COMPARING TAXONOMIC AND MORPHOLOGIC DIVERSITY DURING THE RADIATION OF THE RUMINANTIA (MAMMALIA: ARTIODACTYLA)

Ruminant artiodactyls underwent a dramatic radiation in the Neogene to become one of the dominant groups of large-bodied terrestrial herbivores. Ruminants are an ideal group with which to study taxonomic and morphologic diversity during taxonomic radiations, yet their rise to ascendancy had not been studied in a quantitative manner prior to this study.

In the first part of this study, I used recently proposed survivorship methods to determine the temporal patterns of origination, extinction and preservation throughout the history of ruminants. Additionally, I used phylogenetic tree-based methods to determine if any particular ruminant subclades showed significantly higher rates of diversification.

Results of these analyses inicate two independent radiations driven almost entirely by new origination (and not accompanied by increased rates of extinction as seen in some other taxa), and no intervals of extraordinary extinction. The first is in the Early Miocene and marks the diversification of the Pecora, or "higher" ruminants, which includes many of the extant families of horned ruminants. The second occurs in the Late Miocene and Plio-Pleistocene, and appears to be restricted to the family Bovidae. This corresponds with the radiation of antelope in the Old World.

Patterns of morphological diversity of ruminant dentitions were measured using phenetic disparity (mean pairwise distance) and patristic dissimilarity (mean number of evolutionary steps separating two taxa on a phylogenetic tree). Morphological diversity was also subdivided among ruminant subclades to determine the pattern within subgroups, but also to determine the proportion of overall disparity attributable to each.

Overall ruminant dental disparity shows no significant temporal pattern while patristic dissimilarity increases over much of ruminant history. This suggests appreciable morphological evolution that is not accompanied by an increased range of forms. The relatively constant pattern of phenetic disparity is also in sharp contrast to the persistently increasing taxonomic diversity of ruminants. Subdividing ruminant disparity into different taxonomic components produced few surprises, but suggests that radiation of the Bovidae did involve significant morphological diversification.